Abstract
Abstract. The paper describes a switchable parameterization of collisional ice break-up (CIBU), an ice multiplication process that fits in with the two-moment microphysical Liquid Ice Multiple Aerosols (LIMA) scheme. The LIMA scheme with three ice types (pristine cloud ice crystals, snow aggregates, and graupel hail) was developed in the cloud-resolving mesoscale model (Meso-NH). Here, the CIBU parameterization assumes that collisional break-up is mostly efficient for the small and fragile snow aggregate class of particles when they are hit by large, dense graupel particles. The increase of cloud ice number concentration depends on a prescribed number (or a random number) of fragments being produced per collision. This point is discussed and analytical expressions of the newly contributing CIBU terms in LIMA are given. The scheme is run in the cloud-resolving mesoscale model (Meso-NH) to simulate a first case of a three-dimensional deep convective event with heavy production of graupel. The consequence of dramatically changing the number of fragments produced per collision is investigated by examining the rainfall rates and the changes in small ice concentrations and mass mixing ratios. Many budgets of the ice phase are shown and the sensitivity of CIBU to the initial concentration of freezing nuclei is explored. The scheme is then tested for another deep convective case where, additionally, the convective available potential energy (CAPE) is varied. The results confirm the strong impact of CIBU with up to a 1000-fold increase in small ice concentrations, a reduction of the rainfall or precipitating area, and an invigoration of the convection with higher cloud tops. Finally, it is concluded that the efficiency of the ice crystal fragmentation needs to be tuned carefully. The proposed parameterization of CIBU is easy to implement in any two-moment microphysics scheme. It could be used in this form to simulate deep tropical cloud systems where anomalously high concentrations of small ice crystals are suspected.
Highlights
In a series of papers, Yano and Phillips (2011, 2016) and Yano et al (2016) brought the collisional ice break-up process to the fore again as a possible secondary ice production mechanism in clouds
This step is devoted to the microphysics tendencies of the ice mixing ratios in Figs. 9–11 to assess the impact of the collisional ice break-up (CIBU) process
4.3 Sensitivity to the ice thickness. This last analysis is concerned with the ice thicknesses computed as the integrals along the vertical of ρdref rx, where rx refers to the mixing ratio with x ∈ i, s, g standing for the cloud ice, the snow aggregates, and the graupel hail, respectively
Summary
In a series of papers, Yano and Phillips (2011, 2016) and Yano et al (2016) brought the collisional ice break-up (hereafter CIBU) process to the fore again as a possible secondary ice production mechanism in clouds. According to a concluding remark by Vardiman (1978), this secondary production of ice could lead to concentrations as high as 1000 times the natural concentrations of ice crystals in clouds that would be expected from heterogeneous nucleation on ice freezing nuclei Another laboratory study by Takahashi et al (1995) revealed a huge production of ice splinters after collisions between rimed and deposition-grown graupel. This study does not focus on cloud conditions that lead to explosive ice multiplication due to mechanical break-up in ice–ice collisions Nor does it attempt to reformulate this process on the basis of collisional kinetic energy with many empirical parameters, as proposed by Phillips et al (2017), or earlier by Hobbs and Farber (1972), in terms of their breaking energy, mostly applicable to bin microphysics schemes. A conclusion is drawn on the importance of calibrating the parameterization of CIBU and the need to systematically include CIBU and other ice multiplication processes in bulk microphysics schemes
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